IEEE Transactions on Nuclear Science, Vol. NS-32, No. 2, April 1985
ELECTRON BEAM SIMULATION OF PULSED PHOTON EFFECTSIN ELECTRONIC DEVICES AT VERY HIGH DOSES AND DOSE RATES
A. J. Smitha,bG. Smfitha
W. BeezholdcL.D. PoseyC
V.A.J. van LintdT. F. Wrobel e
Large high-energy flash X-ray simulation facilitiesare expensive to build and operate. As a result, theradiation effects community has at it's disposal alimited number of X-ray sources with the capabilityof providing the very high levels of radiation(hundreds of k rad(Si)) required for R and D.Because of the inefficiency of bremsstrahlungproduction, an accelerator which provides only smalldoses in the X-ray mode could readily provide thevery high total doses and associated dose rates viadirect electron irradiation. A prerequisite forelectron beam testing is a satisfactory demonstrationof the fidelity of the simulation. This paperpresents the experimental details and results of suchfan assessment.
It was demonstrated in this work that electron beamsdo simulate the effects of high-energy bremsstrahlungX-rays when testing semiconductor devices for veryhigh dose and dose rate effects. However, it wasalso found that the effects of charge deposition fromthe electron beam can dramatically perturb thenominal irradiation bias conditions. In electronicdevices where radiation induced degradation is afunction of applied potentials (e.g., MOS devices),this charge capture can totally invalidate thesimulation unless the experimenter is aware of andcompensates for the effect.
Introdugtion
There are many flash X-ray machines available fortransient radiation effects studies in electronicdevices when radiation level requirements are modest,i.e., a few k rad(Si) total dose and dose rates up to1011 rad(Si)/sec. At these levels there are severalsources which can provide an acceptable X-rayspectrum for TREE and IEMP testing over reasonablevolumes (>103 cm3) with good spatial uniformity(-30%). As total dose and dose rate requirements areincreased, the inverse square law can be used toassist simulation but only at the expense of volumeand/or uniformity. Above doses of 100 k rad(Si)there are a limited number of facilities available,e.g., Aurora, HERMES II, and PITHON. These are thelarge and expensive medium-to-high voltage (1.5-12MeV) machines which, especially in the case of thefirst two, exploit the increasing bremsstrahlungefficiency with increasing incident electron energy,thus achieving very high doses and dose rates.[1J
a Atomic Weapons Research Establishment,Aldermaston, Reading, England.
b Now with Ktech Corporation, Albuquerque, NM 87110
c Sandia National Laboratory, Albuquerque, NM 87185
d Mission Research Corporation, San Diego, CA 92123
e Now with "c" above.
However, modest-sized pulsed electron beam machineshave sufficient electron beam energy to provide veryhigh doses and rates, with direct electronirradiation, over quite large areas, to well abovethe highest envisaged levels that electronics can beexpected to survive. However, because of the shortelectron ranges, this simulation technique is onlyapplicable for testing single devices or two-dimensional sub-systems and systems. Electron linearaccelerators (LINACS) have in fact been used for manyyears to provide dose rate simulation on electroniccomponents.
There were two reservations regarding theextrapolation of LINAC or other moderate fluenceelectron techniques of X-ray simulation to the veryhigh dose pulsed electron beam regime. Theseconcerns were:
(1) Charge injection and resulting currentflow, at total dose levels and timescales of interest, could poseproblems by perturbing bias conditionsand/or coupling sufficient energy todamage sensitive devices (e.g., LSI,VLSI).
(2) Degradation of various deviceparameters of some technologieshave exhibited a different functionaltotal-dose dependence for differentirradiation modes. These variationswere more than a dosimetry problem asno simple dose scaling factor could"overlay" the parameter variations.In short, previous data implied thatthe underlying damage mechanisms werem o d e (photon or electron)dependent. [2]
To quantify the limitations, if any, of this electronbeam simulation mode, a joint experimental programbetween the UK and US was initiated. Although onlythe basic experimental simulation fidelity issues arereported here, other aspects of the technique wereinvestigated (dosimetry and electron fluencecontrol). These details have been reportedelsewhere. [3]
A test matrix was devised which was limited to singleactive devices. Included were discrete andintegrated (to LSI) devices, MOS and bipolar, plusdigital and analog types. Each experiment wasconducted at various irradiation levels in the X-rayand the electron-beam modes. Defined parameters ofinterest were either monitored during the pulse orthe pre- and post- irradiation values were recorded.The intention of the study was not to look for subtledifferences between modes but to identify majordifferences. It was anticipated that any majordifferences would not be readily masked by theexpected dosimetry errors or statistical variationsbetween devices undertest.
The concept of using electron beams to simulate theeffects of high energy photons on semiconductordevices is not new. Much work was conducted in thelate 60's and early 70's [4,5,6]. The work waslimited to devices available at the time (up to SSI)and only addressed dose rate effects. This latterpoint is not surprising as the technologies thenunder study (bipolar) are insenstiive to total doseinduced changes. Also, the available doses in the X-ray mode were limited by machine capability [4,5] toabout two k rad(Si). This output limitation set anupper limit on direct mode comparison. The reportedinvestigations at higher doses and rates relied onextrapolation of the limited X-ray data.
It was the intention of the present work to extendthe range of experimental overlap between X-rays andelectrons up to and above one hundred k rads(Si) indevices currently available.
Facilities Employed
The majority of irradiations were carried out on theHERMES II facility at Sandia National Laboratory(SNLA), Albuquerque, NM. HERMES II had thecapability of providing a sufficiently high energyelectron beam (10 MeV Peak) to assure packagingpenetration of all test samples and a uniform dosethroughout the active regions of the devices. Ofgreater importance, this simulator had, when operatedin the pinched electron beam bremsstrahlung mode, thecapability of providing a photon fluence ofsufficient intensity to give total dose in siliconapproaching 0.5 megarad, albeit over small areas(1lcm2). This pinched beam mode provided thereference X-ray response of the devices undertest.These high level X-ray data were used then as abaseline to compare to the electron beam irradiationsconducted on HERMES II and the other electron beammachines.
The other electron beam machines utilized were MINI-Cat AWRE, U.K., (1.5 to 2.2MeV peak energy) and HiFXat HDL, Md. The latter was primarily used for beamdivergence studies (apertures, drift distance anddrift pressure) the results of which are reportedelsewhere. [3]
Experiments Undertaken
The experimental program was designed to test avariety of hardened and unhardened semiconductorparts exposed to X-ray and electron irradiation.Five separate experiments were prepared and these aredescribed below.
employed to allow the threshold voltage data to berecorded by a single oscilloscope pre- and post-test from 10-5 sec to 103 sec (in a 1-3-10 . . .sequence). The upper time limit was determined bymachine repetition rate. This circuit is shown inFig. 1. The device was irradiated in the unpoweredstate as it was anticipated that this condition wouldprovide the greatest sensitivity to electron-beamcoupling (charge capture).
TDOE DIODE DEVICE UNDER TEST
TRIP & TWl3 RAING A i ;'.KVN
UNOTASI3NVL DRIVE UNIT
l VACUUM FEEDROUGH
DELAYED
'5.
FIGURE la. TEST CIRCUIT FOR MOS INVESTIGATIONS.
VAAI ,
V sI1LU II
ki I
I 1 11 MODULATO
I XaCONSTANT
s X'AXI ii
I SIGNAL DELAYED TO ALLOW FOR CABLE PROPAGATION DELAY
FIGURE lb. TEST CIRCUIT WAVEFORMS.
Experiment _ne
A small signal diode (lN916) was irradiated under areverse bias of 50v to determine and comparephotocurrent generation in each test mode. Althoughno discrepancies were envisaged prior to testing,this experiment was aimed at demonstratingequivalence at these high dose rate levels. Inaddition, this simple diode experiment was expectedto yield a noise comparison between test modes.
The radiation-induced threshold voltage shifts ofa (V)MOS transistor (VN66AK) were measured as afunction of dose and time after irradiation. It wasintentional that this device did not have the addedcomplexity of gate protection. A novel technique was
Experiment Three was essentially Experiment Tworepeated on the N- and P-channel devices of ahardened SSI CMOS IC (SLA 4007). The interrogationcircuitry was basically the same as for Two with anadditional channel for P device characterization.
Experiment Fr
A bipolar operational amplifier (pA715) was tested ina simple circuit configuration giving a closed loopgain of ten with an input of zero volts as shown inFig. 2. Parameters of interest which were measuredwere, maximum output voltage excursions and time forrecovery to within 5OmV of the quiescent output value(0 volts).
200-
0
IL
ILa
FIGURE 2. OPERATIONAL AMPLIFIER TEST CIRCUIT.
DOSE (RAD SI)
DATA POINTS ARE FROM ELECTRON BEAM IRRADIATIONS
EXCEPT WHERE SHOWN.
FIGURE 3. PHOTO CHARGE AS A FUNCTIONOF DOSE FOR 1N916 DIODE.
Experiment Five
A LSI hardened CMOS 1802 microprocessor was tested inboth irradiation modes. As with Experiments Two andThree the device undertest was unpowered at the timeof irradiation. Power was then applied to the testdevice at 10 milliseconds after the irradiation andthe device "exercised" with a 1.5 millisecond testprogram. This test program was repeated at eachdecade of time. Correct operation or failure of eachprogram run was remotely monitored.
Test Configurations and Dosimetry
Although several dosimetry systems were fielded,standard SNLA TLD's (CaF:Mn) were used for reference
purposes in this work. With the exception ofExperiment Five, which demanded a large number ofvacuum feed-throughs, a single test enclosure was
employed. The chamber was used in ambient air forthe X-ray exposures and in vacuum for the electronbeam irradiation. The chamber included a screenedenclosure of about 7.5 cm diameter and 5 cm depthwith a one mil aluminum front face. The signal feed-throughs were screened from the incident electronbeam with aluminum. All external cables were
provided with -5cm of lead to minimize any X-ray-induced cable effects.
machine the two modes of irradiation had different
widths.
The data show, within experimental error, that diode
photocurrent generation is independent of machine
characteristics (pulse width, end point energy, i.e,
spectrum) and mode of simulation.
ExperinmentTw
The data from the oscillograph record were analyzed
using the usual MOS formula:
IDS = A(VGS - VTH)2. (1)
where: VGS VGG - Vs and:
IDS = Vs/RM : RM = effective monitor resistance(10Q in parallel with 50(1)
A least-squares fitting routine was used to extract
the best values of threshold voltage, VTH, for eachset of data points. A condensed set of data, that of
threshold voltage shift one second after irradiation,
is shown in Fig.4. Data at all other times show the
-201
Experiment Results
ExDeriment One
The diode photocurrent experiment was conducted on
both HiFX and HERMES II. No problems were
experienced when utilizing the X-ray mode. However,there was evidence of signal noise in the electron-beam test. This noise was eliminated by using two
current transformers in a push-pull differential
arrangement. The transformers were geometricallyarranged such that extraneous noise was self
cancelling. Figure 3 shows the measured radiation-
induced charge flow as a function of total dose onthe two machines each operated in both modes. The
coordinated system was chosen to make the data moreinsensitive to pulse-width differences. Pulse widthinfluences peak current measurements through the
-error-function response of the diffusion component of
photocurrent. Not only was there a factor of -2.5between the nominal pulse widths of the simulatorsused (HiFX 25 nsec:HERMES 60 Nsec), but also on each
-is0
tow
0 -10
0
0$ -S
+ X-RAY IRRADIATION
+ ELECTRON IRRADIATION
+~~~~~~~
@8+- 4
+
100 200
DOSE (K RAD(SM))N00
FIGURE 4. VN66AK THRESHOLD VOLTAGE SHIFT AT 1SECOND AS A FUNCTION OF TOTAL DOSEAND MODE OF IRRADIATION.
1200
1~~~~~-I31
same trend. The values of threshold voltage shiftsdetermined from the recorded data through equation 1were accurate to within ±5OmV. However, Fig. 4 errorbars represent the calculated maximum devicevariation assuming the effect is proportional to lihesquare of oxide thickness (1500 A ± 200 A).Dosimetry errors were estimated to be ±10%. Despitethese large uncertainties it is apparent thatelectron beam irradiations produced a significantlygreater effect in these transistors than X-rays.
Terimen Tlhree
The data were analyzed as above. The results areshown in Fig. 5. In the case of the 4007 there is noapparent difference in response between electron andphoton deposition modes.
-0.6
U .e
0
o 0.40ut
-* 02
U00
,ELECTRON BEAM ' CHANNELA ELECTRON BEAM 'N' CHANNEL
o X-RAY MODE 'P'CHANNELX-RAY MODE 'N' CHANNEL*I -A OE N HNE
_
0
.
0
° AA
I * * I I
20 40 60 60TOTAL DOSE (KR Sl)
100 120
FIGURE 5. SNLA 4007 'N' AND 'P' CHANNEL THRESHOLDVOLTAGE SHIFT AT 1S VERSUS TOTAL DOSE.
1201
to eliminate charge capture, which could cause devicefailure, by incorporating a lead shield (thickness >max. electron range) covering all of the test fixtureexcept the 1 cm2 active area of the test device.There was sufficient coupling to cause failure of theinterrogation system. It was observed that alldevices that had shown failure at 10-3 sec did notrecover during the interrogation period. In anattempt to quantify electron coupling it wasconsidered worthwhile to irradiate the final device.This final experiment was conducted at the samefluence as the previous one. Now, however, theactive chip was shielded with lead but the testfixture was not shielded.
Upon return to the UK all devices were againelectronically exercised. Each device which failedto function at this time was annealed for ninetyminutes at 2500C and retested. A summary of theresults is shown below.
DeviceDose Test ResultK rad
Device Mode (Si) A B C Comment
1 X-ray 60 P P2 n 400 F F P3 n 400 F F P
4 Electron 20 F F F5 n 15 X P Socket Screened6 n 2.0 X F P Device Screened
P = Pass F Fail X = Don't KnowA = 10-3 to 10+3 sec after IrradiationB 1l week after IrradiationC = B + 90 mmn Anneal at 2500 C
The results showed no significant difference inresponse, up to the maximum dose common to bothmodes. Voltage excursions were between +0.8 and -0.6volts on each test. Malfunction time averages werenoted as below:
CorruptionTime
Mode Microsecs_________ ---------
X-rayElectron
1720
StandardDeviation
+1.5
+9 .0
As the root cause of corruption is the flow ofphotocurrent this finding is consistent with that ofExperiment One.
Experiment iv
The test sample size available for this experimentwas restricted to six devices. Each irradiation modewas allocated three devices. On the first X-ray testthe 1802 survived at 60 k rad(Si). Unfortunately,the two final irradiations in the X-ray mode (equaldoses of 400 k rad(Si)) caused the devices to ceasefunctioning and thereby gave a wide go-no go window.The first device irradiated in the electron beam to20 k rad(Si) failed. Active measurements indicatedabout 20 volts of noise induced on the power supplyline of the test fixture. From this information itwas assumed that charge collection was the cause ofthis failure. The next test configuration attempted
From the limited data, the results are consistentwith the findings of Experiment Two, i.e., theeffects of electron beams at high doses and rates aremore severe than those arising from photonirradiations. The electron-beam data suggest thatcharge capture by the large test fixture causespotentials at gate oxides to be present duringirradiation. This bias increases positive chargebuild-up and thereby produces larger thresholdvoltage shifts. In device four this inducedpotential caused catastrophic burn-out.
Follow-UD Work
Although electron capture is a plausible explanationfor the "anomalies" observed in Experiments Two andFive, the evidence was nevertheless circumstantial.Figure 6 depicts the proposed mechanism for theenhanced shift in the electron irradiation mode. Thedevice header (T039), although only a fraction of an(HERMES II) electron range thick, will stop someelectrons, thus causing the drain to drop inpotential. At sufficiently high dose rates thedrain-substrate diode turns on and a potential iscreated across the gate oxides, (the RC time constantof the gate is approximately one nanosecond, so thegate stays essentially at ground). This potentialgives rise to an increased shift in thresholdvoltage.
An alternative but remote possibility with farreaching consequences would be that electrons, perse, do not simulate X-ray effects in some cases forsome devices. This latter possibility was addressed
1202
-14 r
iiV0 (t.0) AFTER 75 K RAD(8)ELECTRON IRRADIATION (HERMES)
GATE-SOURCE VOLTAGE
DURING IRRADIATION (VOLTS)
FIGURE 6. EXPERIMENTAL ARRANGEMENT FORDEVICE UNDER TEST.
first. Experiments were undertaken on EROS (X-Rays),on a LINAC (10 MeV electrons) and, cyrogenically (LIQN2), on a 6°Co source using similar devices. Theresults are given in Fig. 7. These results show that
5.0 r
- 4.0
0
co
-.
0
- 1.0Luo0
2 .0
L - LINAC (ELECTRONS)
E- EROS (x-RAYS)
--- COBALT 60
- LI
- -4-0~,- -+ -
e$ -- >- ~ o
"
FIGURE 8. 75 KR(SI) COBALT 60 (LIQUID NITROGEN)THRESHOLD VOLTAGE SHIFT AS AFUNCTION OF GATE-SOURCE VOLTAGESHIFT (POINTS OBTAINED BYINTERPOLATION).
potential of between 3.2 to 4.3 volts. These twoestimates are self-consistent.
Experimental confirmation was still required. Amodification to the standard test circuit was neededwhich would prevent any gate supstrate potentialdifference from developing during electron-beamirradiation. Several mechanical and electronicschemes were tested. By far the best in benchtesting in high frequency (nano-second) performanceis shown in Fig. 9. Electron-beam irradiations were
I1 I
10 1 5 20 30 40
TOTAL DOSE (KR(SI))
FIGURE 7. THRESHOLD VOLTAGE SHIFT IN VN66AKAT ONE SECOND AS A FUNCTION OFTOTAL DOSE. ALSO SHOWN IS THRESHOLDVOLTAGE SHIFT INDUCED BY COBALT 60AT LIQUID NITROGEN TEMPERATURES.
all irradiation modes at lower doses and dose ratesmodes give, within expected error, the same results. FIGURE 9. MODIFIED TEST CIRCUIT TO ELIMINATE
INDUCED GATE-SUBSTRATE POTENTIALS.
Ih_q In-Situ Electron-Beam Bias Effect
The above-mentioned electron-induced bias hypothesiswas tested theoretically and experimentally. First,the TIGER electron transport code was used to obtaindose/charge depositon in the test devices andelectron irradiation chamber. TIGER predicted that,per incident electron, the dose in silicon was 2.3MeV/gm and electron capture in the header was 0.13(electrons captured per inqident electron). Aneffective capture area of 0.6cm2 (T039), theresistance values shown in Fig. 7, and a dose of 75 krad(Si) deposited in 50 nsec (a data point on Fig.
4), produces an induced potential of 3.2 volts.
Figure 8 shows data (interpolated) from liquid
nitrogen 60Co irradiations. These data indicate that
the measured electron-beam irradiation thresholdshift at 10-5 seconds would require a gate-substrate
performed on MINI-C with and without themodification. This machine, having a lower energyelectron beam, gave rise to a more pronounced effect.Component availability dictated that 2N6660 deviceswere used in place of the VN66AK. (These arenominally identical devices; it was noted, however,that the new device exhibited a much reducedannealing rate). Figure 10 presents the measureddata for the two circuit configurations. Dosimetryerror bars are +10% while vertical bars indicatemeasured shifts from 10-4 sec to 10+3 sec and arecentered on 1 second. Also plotted on this figure isthe 60Co shift as a function of total dose with upperand lower limits based on
AV-t2 where t2 1500 + 200A.x ox
_-12
0
Z -10
0oIL
O -80coI-
60 s0
IF
F
1203
Acknowledgement
+ WITHOUT CLAMPINGDIODE
+ WITH CLAMPING DIODE
The authors wish to acknowledge the contributions ofA. Moat; Dr. E. Conrad (then at the Defense NuclearAgency (DNA) Headquarters), Dr. J.E. Powell at SandiaNational Laboratories (SNL) and Dr. E.M. Gunnerson atAWRE for encouraging collaboration between DNA, theUK AWRE, and SNLA to study electron-beam TREEsimulation and for funding the work summarized inthis paper; and Dr. B.L. Gregory for supplying anumber of the semiconductor devices used in thisstudy.
COBALT 60
I-9-
0 50 100
TOTAL DOSE (K RAD(SI))
FIGURE 10. THRESHOLD VOLTAGE SHIFT(2N6660) WITH AND WITHOUTCLAMPING DIODE AS A FUNCTIONOF TOTAL DOSE DELIVERED BYPULSED ELECTRON BEAM.
Conclusions
It is apparent that the anomalous results obtained inelectron-beam irradiation arise as a directconsequence of the parameter under investigationbeing some function of total dose and biasconditions. While electron beams simulate the dose,they can readily perturb the bias and thereby themeasured result. The 4007, although MOS, did notexhibit an identifiably different response betweenmodes due to the diode input protection network thatprevented (as in the modified 2N6660 experiments) anyelectron-induced voltage exclusions.
In the case of the 1802 IC, charge capture was a moresevere problem arising from the 40 lead test fixture.The multi-lead charge collection, we presume, causeddevices to fail at a lower level of irradiation inthe electron mode than in the X-ray mode.
Electron beams can be used to simulate high X-raydose rates. However, the experimenter must exercisecaution and ensure that the effects of charge capturedo not invalidate the results. Acceptable chargecapture levels could be demonstrated during testingby real-time monitoring of induced voltage excursionson sensitive circuit nodes.
[1] T.W.L Sanford and J.A. Halbleib,"Radiation Output and Dose Predictions for FlashX-ray Sources,"This Conference.
[2] R.F. Richards, S.R. Slater & S.F. Calaco,"An Assessment of the Radiation Tolerance of theCollector Diffusion Isolation BipolarTechnology," T-NS v 26 nl4 pp. 952-958, Feb. 79.
[3] T.F. Wrobel,"High Dose Rate Electron Beam Testing",MRC/SD-R-68, Dec. 1980.
t4 ] J.S. Nichols, Dr. Alexander & A.H. Hoffland,"Active Measurements in the Electron Beam of theAFWL 2MeV Field Emission Flash X-ray Machine",AFSWC-TR-5, Vol III, March 70 pp. 47-64
[5] J.L. Andrews and J.E. Tarka,"Recent Developments in Transient RadiationEffects Testing Utilizing Pulsed ElectronBeams",Ibid p. 65-96
[6] R.B. Oswald, Jr., H. Eisen, E. Conrad,"Pulsed Electron Beam Dosimetry",IEEE Trans NucSci, NM-13, p. 220-236, Dec. 66
Paper Dresented at the NPSS 1984 Annual Conference
and Space Radiation Effects, Colorado Springs CO,
July 23-25.
In the case of single discrete devices, experimentalconfigurations can be defined to overcome the effect.For simple circuits there may be conflictingrequirements such that pulsed electron beam testingcan never be employed. Each case requires analysisto demonstrate that simulation would be valid.
Finally, since vast amounts of data have beengenerated on LINACS, it is worthwhile performing therelevant calculation of electron capture and itsensuing autobiasing. Assuming a beam energy of 10
MeV and dose rate of 1010 rad(Si)/sec on thegeometry employed above, then the drain drops inpotential by about 6OmV. This is insufficient toturn on the drain-substrate diode and hence 10MeVLINAC testing at 101 0 rad(Si) creates no gate oxidebias. Existing data would thus be valid. When doserates are increased above 1011 rad(Si)/sec, testresult validity would become increasingly dubious.
